Elsevier

Icarus

Volume 219, Issue 2, June 2012, Pages 689-700
Icarus

Constraints on Saturn’s tropospheric general circulation from Cassini ISS images

https://doi.org/10.1016/j.icarus.2012.03.035Get rights and content

Abstract

An automated cloud tracking algorithm is applied to Cassini Imaging Science Subsystem high-resolution apoapsis images of Saturn from 2005 and 2007 and moderate resolution images from 2011 and 2012 to define the near-global distribution of zonal winds and eddy momentum fluxes at the middle troposphere cloud level and in the upper troposphere haze. Improvements in the tracking algorithm combined with the greater feature contrast in the northern hemisphere during the approach to spring equinox allow for better rejection of erroneous wind vectors, a more objective assessment at any latitude of the quality of the mean zonal wind, and a population of winds comparable in size to that available for the much higher contrast atmosphere of Jupiter. Zonal winds at cloud level changed little between 2005 and 2007 at all latitudes sampled. Upper troposphere zonal winds derived from methane band images are ∼10 m s−1 weaker than cloud level winds in the cores of eastward jets and ∼5 m s−1 stronger on either side of the jet core, i.e., eastward jets appear to broaden with increasing altitude. In westward jet regions winds are approximately the same at both altitudes. Lateral eddy momentum fluxes are directed into eastward jet cores, including the strong equatorial jet, and away from westward jet cores and weaken with increasing altitude on the flanks of the eastward jets, consistent with the upward broadening of these jets. The conversion rate of eddy to mean zonal kinetic energy at the visible cloud level is larger in eastward jet regions (5.2 × 10−5 m2 s−3) and smaller in westward jet regions (1.6 × 10−5 m2 s−3) than the global mean value (4.1 × 10−5 m2 s−3). Overall the results are consistent with theories that suggest that the jets and the overturning meridional circulation at cloud level on Saturn are maintained at least in part by eddies due to instabilities of the large-scale flow near and/or below the cloud level.

Highlights

► Saturn eastward jets broaden with increasing altitude. ► Eddy momentum flux into eastward jets, decreases upward near eastward jets. ► Implies jets are driven by processes acting within the middle troposphere clouds.

Introduction

One of the primary objectives of the Cassini Orbiter mission to Saturn has been to document Saturn’s general circulation and to provide observational constraints on theories advanced to explain the circulation. Prior to Cassini, Saturn was less comprehensively observed than Jupiter, but 4 years of data from the Cassini nominal mission and new data from the extended mission now allow us to place Saturn on at least an equal footing with Jupiter despite the greater observational challenges presented by Saturn’s multi-layer hazes and clouds.

The history of observations of Saturn’s dynamics and the evolution of our understanding of the relevant processes up through the early years of the Cassini mission is discussed in the review by Del Genio et al. (2009). Like Jupiter, Saturn’s major observed dynamical feature is its series of alternating eastward and westward jets, although whether Saturn actually has westward jets or merely eastward wind minima depends on the poorly known rotation rate of the deep atmosphere (Desch and Kaiser, 1981, Anderson and Schubert, 2007, Read et al., 2009a, Read et al., 2009b). The past few years has seen considerable progress in numerical modeling of jovian planet atmospheres targeted at understanding possible mechanisms for the observed jet structure. The modeling efforts fall into two broad categories – those that emphasize processes occurring in shallow weather layers near the visible cloud level (e.g., Lian and Showman, 2010, Liu and Schneider, 2010), and others that explore the ramifications of processes that extend through the deep molecular hydrogen envelopes of the jovian planet atmospheres (e.g., Heimpel and Aurnou, 2007, Kaspi et al., 2009). It has often been assumed that observations of the vertical structure of the wind itself differentiate these two classes of theories. However, it is possible for shallow forcing to influence the dynamics at depth and vice versa, so knowledge of the vertical structure of the mean zonal wind provides a valuable observational constraint but must be accompanied by other constraints to identify the relevant mechanisms (Showman et al., 2006, Del Genio et al., 2009).

The Cassini Imaging Science Subsystem (ISS; Porco et al., 2004) is one of the primary sources of information available to document dynamical processes operating in Saturn’s atmosphere. The narrow-angle camera (NAC) allows individual regions on Saturn to be imaged at high resolution near distant apoapses, making hemispheric or near-global mosaics possible, while the accompanying wide-angle camera (WAC) provides simultaneous global views required for image navigation. ISS observes Saturn from the ultraviolet to the near-infrared, including both continuum and methane band filters, which produces information on clouds and hazes from Saturn’s middle troposphere to stratosphere.

ISS images of the southern hemisphere of Saturn from the Cassini approach to Saturn and from early orbits at mostly moderate resolution have been used to document the latitudinal profile of mean zonal wind and its changes since the Voyager and Hubble eras (Porco et al., 2005, Vasavada et al., 2006, Sánchez-Lavega et al., 2007, García-Melendo et al., 2009); later images provide global coverage (García-Melendo et al., 2011). These studies primarily used either a manual tracking or a one-dimensional line shifting correlation approach. Manual tracking provides the highest confidence in individual wind vectors but its sampling is limited and the results can vary due to the subjective selection of targets. Line-shifting methods using a large segment of a latitude circle allow for very high latitudinal resolution but can only provide information on mean zonal winds, while potentially being influenced by large-scale waves whose phase speeds differ from the speed of the underlying wind (e.g., Del Genio and Rossow, 1990).

Del Genio et al. (2007) applied an automated cloud tracking approach to Saturn using a two-dimensional feature correlation method. The primary advantage of this method is that it uniformly samples an entire image in small areas corresponding to one or a few individual cloud features. It thus provides a more objective estimate of the actual mean wind (to the extent that the vectors that satisfy quality criteria are not a spatially biased sample). Furthermore it allows higher order statistics such as eddy momentum fluxes to be calculated since it yields two-dimensional information on the wind field at any level (three-dimensional if multiple filters are used). Because the features tracked are two-dimensional, though, it provides lower latitudinal resolution than one-dimensional techniques, and because it is automated it is subject to erroneous estimates of the wind in areas with little albedo contrast or mostly linear features.

Del Genio et al. (2007) applied the automated method to ISS Saturn near-infrared continuum filter images from early in the Cassini nominal mission. Because of the limitations mentioned above, we were only able to retrieve reliable wind estimates at some latitudes, the most problematic regions being the latitudes of the eastward jet cores. The early images also covered only the southern hemisphere due to obscuration of the northern hemisphere by the rings, ring shadows, and lack of insolation. Nonetheless the technique was good enough to provide an order of magnitude more sampling than was possible with the Voyager images (Ingersoll et al., 1984), while also allowing for the first credible estimates of the eddy momentum flux distribution on Saturn.

In this paper we extend our results to the 2007 time period, when Cassini was placed into a series of large daytime apoapsis orbits at low phase angle designed specifically to optimize large-scale imaging. These orbits give us our first views of the northern hemisphere of Saturn, permitting us to document the global wind characteristics but also giving us sufficient sampling to create regional composites of essential features of the dynamics. Additional recent images (2011–2012) allow us to track more of the previously obscured equatorial region as well. We also describe an improvement in our cloud tracking scheme that allows us to sample more of the eastward jet core regions with confidence, reduce uncertainties in our estimates of eddy momentum flux, and perform the first fully automated tracking of individual cloud feature areas in Saturn methane band images. Section 2 describes the data and methods used as well as several sensitivity tests of the algorithm. Section 3 documents the mean wind and eddy momentum flux distribution at a continuum wavelength, while Section 4 compares these to similar estimates at a methane band wavelength. Section 5 discusses the implications of our results for dynamical theories of jovian planet jet maintenance and the meridional overturning circulation.

Section snippets

Data selection and processing

The primary dataset used consists of ISS NAC continuum band (CB2; 750 nm) high-resolution (∼17–23 km pixel−1) images acquired on one early orbit in 2005 (Rev 3) and 3 of the later large dayside apoapsis orbits in 2007 (Revs 48, 49, 52). During Rev 48, limitations on data volume due to competition with other instruments for tape recorder space required 2 × 2 pixel summing, reducing the effective resolution. A total of 360 CB2 images taken from 28 mosaic pairs separated by approximately one Saturn

Cloud level (CB2) dynamical features

Fig. 6 shows our complete zonal wind profile for all time periods analyzed. We calculate zonal wind in the traditional manner based on the System III estimate of Saturn’s rotation period (Desch and Kaiser, 1981). In this reference frame what we refer to as “westward jets” are sometimes merely minima in eastward winds rather than actual westward wind maxima. Shorter rotation periods that imply real westward jets and weaker eastward jets have been proposed (Anderson and Schubert, 2007, Read et

Upper troposphere (MT2) dynamical features

Fig. 9 shows the latitudinal profile of mean zonal wind from MT2 images superimposed upon the CB2 profile. With the automated tracking algorithm we are able to compute MT2 winds at all latitudes that we observe in the northern hemisphere; in the southern hemisphere, we are less able to track in the eastward jet cores than is possible in CB2 images because of the generally lower feature contrast and greater linearity of features in MT2 images (Fig. 1). The most obvious difference between CB2 and

Discussion

The first-order questions about the dynamics of Saturn’s atmosphere are whether the mean zonal flow long observed at cloud tops is shallow or deep, and whether the processes driving it are themselves shallow or deep. An extensive discussion of the candidate theories can be found in Del Genio et al. (2009). Shallow “weather layer” models of the flow rely on forcing by radiative heating due to differential insolation or latent heat release due to the condensation of water, while deep flow

Acknowledgments

This research was supported by NASA Cassini Project funding of the Imaging Science Subsystem team. We thank the reviewers for constructive comments that helped improve the manuscript.

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